This invention relates generally to filter networks for communication systems and more particularly to a filter network for separating a communication frequency into discrete frequency channels.
In certain communications systems it is desirable to transmit a plurality of signals over a particular frequency band. Typically, these transmitters employ filter bank separators and combiners which serve to split the transmitters frequency band into smaller frequency segments referred to as channels. The frequency limits of each channel is chosen to meet the needs of the system, whether it be radio, radar, telemetry, etc.
The major problem with the existing channelization approach is that at times it is necessary to change the channel frequency relatively quickly as new needs arise for the system. Some systems, which may be mobile as well as stationary, may require modification in the field, which is ordinarily a long, laborius and expensive operation.
One solution to the problem has been the utilization of plug-in-filters. This approach permits one or more filters to be changed relatively easily without requiring the replacement of the entire separator and combiner filter banks. This method also has limitations which make it less than a completely desirable solution. For example, the plug-in-filter approach requires more circulators than needed to implement a fixed filter bank. As a result the volume required is greater, the cost higher and in some channels, the insertion loss greater. Further, in order to move merely a single cross-over point in a triplexer system without loss of continuous coverage, it is necessary to replace 4 of the 6 plug-in filters used to implement the triplexer. Hence, such a plug-in circuit would constitute only a slight improvement over the system which requires replacement of the entire separator and and combiner filter banks.